Obviously a manufacturer could do anything they want but everything has a cost. Cost affects sales and sales affects keeping retail prices down. Most products get a price fixed to them when they are designed. Replaceable batteries have a cost which means if they add that as a feature then something else will get left out. So decisions get made.
Or, certain standards are upheld and the price is adjusted accordingly. With one exception I can currently think of (Fishman Triple Play), I haven't bought any new musical equipment with a non-user replaceable battery - ever.
There was no other option like the Triple Play and I really wanted it. So far so good, you are right about newer battery systems having fairly long lives if properly charged.
Behringer doesn't have to fix anything, I bought one small mixer from them a long time ago and that ended that. They have plenty of customers, I am not one of them.
Now phones are different subject. Until a friend gave me his extra phone to use for free, I was fine with the Tracfone Android for $30. I am probably an exception to the standard use case, I spent maybe $100 a year for phone service.
So I am an outlier but I find a way to avoid non-replaceable batteries for the most part.
I get what you are saying, I don't have any negative feelings about you stating a reality that exists. Budget, Deliverable, Obstacles - same old story just different players and different potentials as tech evolves.
Some will make stuff I will buy, some won't.